LINEAR ELASTIC FRACTURE MECHANICS THEORY OF THE PLAIN STRAIN AND PLAIN-STRESS FRACTURE MODES .

FRACTURE MECHANICS:-- The post fracture examination of the fracture surfaces generally indicates about the behaviour of the material. This is true not only of the fracture of a precracked body but of an intact Structure of the material. In a tensile test the familiar cup-cone type of fracture is indicative of a ductile tensile failure contrasted to a flat brittle failure. Fracture mechanics has provided a more specific definition to distinguish between brittle and other than brittle failure.Linear elastic fracture mechanics (LEFM) was founded on elastic fracture, i.e the area remote from the crack tip remains elastic and the plastic zone of the crack tip is small .Since as already pointed out, this is not the case in many materials.It becomes important to tie elastic and elasto-plastic fracture together in the confines of fracture mechanics. This tie can be made through the understanding of plain strain and plain stress as related to the fracture. In the majority of plain strain fractures critical parts particularly in aircraft Structures the thickness of the material is generally small and plain strain condition do not prevail and the fracture occurs under plain stress or mixed mode conditions. Experimental results have shown that plain stress fracture significant plastic effects are present and fracture toughness based on linear elastic fracture mechanics does not adequately represent the material behaviour.